Grain size-dependent electrical properties of rutile (TiO2)

C. Demetry and X. Shi, Solid State Ionics118
(1999) 271-279.

Abstract:
The electrical behavior of Sn-doped nanocrystalline and microcrystalline
TiO2 has been studied by DC conductivity measurements and impedance
spectroscopy. Specimens were hot pressed from powders synthesized by a
solution chemical method, resulting in highly dense, 100% rutile with an
average grain size of approximately 50 nm. Results indicate that
nanocrystalline TiO2 has enhanced conductivity relative to
microcrystalline TiO2, and a lower specific grain boundary resistivity.
The activation energy for conduction in air of both nanocrystalline and
microcrystalline TiO2 is lower than values obtained from the literature
for polycrstalline and single crystal rutile. Furthermore, conductivity
measurements over a range of oxygen partial pressures indicate that the
conductivity of nanocrystalline and microcrystalline TiO2 exhibits a
PO2-independent regime that is unique compared to past studies of
polycrystalline and single crystal rutile.

Abstract:
The goal of outcomes assessment is to inform educators about how and what
students are learning. It is an iterative process of defining and
communicating educational goals and objectives, thinnking about criteria
by which we will know when the objectives have been achieved, designing
practices to assist students in meeting our expecations, measuring student
performance against the criteria, and evaluating results and supplying
feedback to appropriate audiences for continuous improvement of the
educational process. In this paper, a general model for outcomes
assessment is described that can be applied to a wide range of educational
goals and activities and for a variety of purposes. Examples of
small-scale classroom assessment specific to materials science educators
are provided and critiqued.

Abstract: Materials selection projects supplied by WPI
alumni in industry were introduced to a large enrollment introductory
materials science course for all engineering majors. The rationale for
introducing the project was multifold: 1) for students to integrate and go
beyond course topics by addressing a real, open-ended materials selection
problem recently faced by an alumnus in industry; 2) to generate more
interest in materials science among students of all majors; and 3) to
improve students' communication and teamwork skills. Projects were
conducted in assigned teams, with checks for individual accountability.
Each team prepared a written report and oral presentation, which together
accounted for 25% of the overall course grade. Assessment of this course
innovation has indicated that the alumni-contributed project topics did
not generate more interest in materials science, in comparison to previous
iterations of the course's project component, which included reverse
engineering "product dissection" projects and instructor-generated
materials selection topics.

Abstract: The use of tactile sensors has
been explored as a novel means of assessing density uniformity of powders
in molding dies prior to compaction. Nonuniform mass distribution after
the die filling process was detected as gradients in the transmitted
pressure by a tactile sensor located at the bottom of the die. The effect
of process variables on fill uniformity was determined by comparing the
coefficient of variation of transmitted pressure distributions. Direct
measurements of density uniformity after compaction were consistent with
the trends in pressure distribution measured by the tactile sensor. Thus,
this work demonstrates the feasibility of using tactile sensors as a
quality assessment, process development, or training tool in dry pressing
operations.

A Comparative Assessment of Students'
Experiences in Two Instructional Formats of an Introductory Materials
Science Course

Abstract: An educational experiment at Worcester Polytechnic
Institute is described in which the instruction of the Introduction to
Materials Science course (ES2001) was modified to incorporate active and
cooperative learning. The overall goal was simultaneously to
enhance educational quality and faculty productivity. Aspects of the course modification included use of "active" rather than traditional lectures, assignment of students to cooperative learning teams, introduction of a "product dissection project," and
a "teacher as manager" approach to instruction, in which undergraduate Peer Learning Assistants and the graduate Teaching Assistant took on responsibilities as part of the instructional team. Data was gathered from 382 students in three traditional course
offerings and two active/cooperative offerings, using various survey instruments to measure students' learning retention and performance, interest in materials science, their attitudes about learning, and their satisfaction with the course.

The WPI-Norton Company Project Center: A University-Industry Partnership in Undergraduate Education

Abstract:
The WPI-Norton Company Project Center, in operation since 1973, is a partnership designed to enhance undergraduate engineering education at WPI and to solve problems of direct relevance to Norton Company. Student teams complete year-long projects on topic
s proposed by Norton Company and are advised both by WPI faculty and Norton employees. This arrangement differs from co-operative education in that the projects satisfy a WPI degree requirement, and the students are not paid by the company. This paper des
cribes some logistical details of the Center's operation and highlights successes and challenges revealed by a recent evaluation of the partnership.

C.D. Terwilliger and Y.-M. Chiang, Journal of the American Ceramic Society78[8], 2045-55 (1995).

Abstract:
Differential scanning calorimetry has been used to make direct measurements of the excess enthalpy of TiO2 (rutile) with an initial grain size of 30-70 nm. When the heat released during grain growth is normalized to the change in grain boundary area, the
specific excess enthalpy at low temperatures and fine grain sizes (600-780 C, 30-200 nm) is found to be 0.5-1 J/m2, while values averaged over a larger temperature and size range (600-1300 C, 30 nm~2um) are 1.3-1.7 J/m2. After exclusion of extraneous cont
ributions from other heat-dissipating processes, origins of a specific grain boundary enthalpy that increases with grain size or temperature are considered, including solute segregation, changes in grain boundary structure, and contributions from grain bo
undary triple junctions. It is concluded that the most plausible explanation is a size-dependent nonstoichiometry of TiO2 due to the impingement of space charge layers in the temperature and grain size range of the experiments.

Abstract:
Grain boundary segregation at ultrafine grain sizes has been studied. Using a STEM microanalysis technique to quantify the grain boundary coverage of calcium (0.34 mole%) in TiO2 ranging in grain size from 50 nm to 750 nm, it is found that below grain siz
es of 150-350 nm, segregation deviates from conventional isotherms, exhibiting a clear size dependence. In this size regime the interfacial area to volume ratio is as important as temperature and composition in determining grain boundary coverage. In the
present system, grain boundaries become saturated with calcium when the coverage reaches approximately one half of an equivalent monolayer. The experimental results can be modeled by a statistical thermodynamical treatment of segregation which takes into
account the large density of grain boundary sites in this size range. We also find direct evidence of enhanced total solubility at very fine grain sizes due to grain boundary segregation.

Abstract:
Differential scanning calorimetry and transmission electron microscopy are used to monitor grain growth in nanocrystalline TiO2 doped with Sn and Ca. TiO2 powder is prepared by a solution chemical method, and consolidated samples have an initial mean grai
n size of 30-50 nm, with a density greater than 95% of theoretical. Calcium is an effective grain growth inhibitor; the present samples show more resistance to grain growth than other nanocrystalline TiO2 reported to date. A Kissinger analysis of grain gr
owth exotherms indicates an effective activation energy of 1.3-1.5 eV, and calcium additions do not have a large effect on this value. These observations are discussed in light of the size-dependent segregation behavior that has been observed in these sam
ples using STEM microanalysis.

Abstract:
A solution-based processing method has been used to synthesize nanocrystalline TiO2 powder. By doping with a small amount of Sn, the rutile phase is formed with an average grain size <20 nm. These powders can be hot-pressed into pellets of 95% theoreti
cal bulk density while maintaining an average grain size between 30-50 nm. Other pellets have been prepared similarly using powder synthesized by the inert gas condensation process. Here we report on the characterization of both types of nanophase TiO2 by
x-ray diffraction and transmission electron microscopy. While the physically-derived samples have somewhat finer grain sizes, the Sn-doped chemically-derived samples are more readily densified, appear to be highly stoichiometric, and are free of the anat
ase phase.